<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
      

        ul {
            display: flex;
            position: relative;
        }

        li {
            margin: 0;
            padding: 0;
            list-style: none;
            width: 100px;
            height: 50px;
            background-color: blue;
            text-align: center;
            line-height: 50px;

        }

        li:hover .box {
            height: 50px;
            opacity: 1;
            /* display: block; */
        }

        .box {
            position: absolute;
            top: 50px;
            left: 40px;
            width: 400px;
            height: 0;
            opacity: 0;
            background-color: red;
            transition: all 0.3s 0.1s;
/* display: none; */
        }
    </style>
</head>

<body>

    <div class="bigbox">
        <ul>
            <li>
                <span>吃饭</span>
                <div class="box">
                    吃好吃的
                </div>
            </li>
            <li>
                <span>睡觉</span>
                <div class="box">
                    睡觉
                </div>
            </li>
            <li>
                <span>打豆豆</span>
                <div class="box">
                    打豆豆
                </div>
            </li>
            <li>
                <span>我是豆豆</span>
                <div class="box">
                    我是豆豆
                </div>
            </li>
        </ul>
    </div>

</body>

</html>